1. Technical Field of the Invention
The present invention relates to an electro discharge (or electric discharge) machining apparatus used for a micro-machining process, for example, in the formation of micro holes or slits, by utilizing electro discharge generated (or electric discharge) generated in a micro-discharge gap between a tool electrode and a workpiece.
2. Description of Related Art
Micro-electro discharge machining techniques have been used mainly for drilling micro holes for e.g. the nozzles of an inkjet printer. In response to the demands in recent years, ultra-micro electro discharge machining apparatuses capable of drilling holes of as small as 5 xcexcm in diameter have been developed. Such apparatus has an overall construction as shown in FIG. 7. Micro-electro discharge machining apparatus of such kind comprises a machining head 1 that is equipped with a tool electrode 4 and moved up and down, a positioning mechanism 2 which controls the movement of a workpiece 7 and positions same with respect to the tool electrode 4, and a control unit 3 including a discharge circuit 5.
The machining head 1 comprises a mandrel 8, and the tool electrode 4 is mounted to a lower end of the mandrel 8 so as to extend vertically downwards. The tool electrode 4 is driven to rotate by a rotation drive power source (not shown) such as a D/C motor, and is fed precisely in the machining direction, i.e., vertically downwards, by means of a ball screw (not shown) that is rotated by a motor 9. The tool electrode 4 is rotated around its axis for the purpose of removing chips produced by machining and achieving true roundness of the tool electrode.
The positioning mechanism 2 comprises a tank 11 filled with dielectric 10 composed of an insulating fluid such as pure water, a table 12 disposed inside of the tank 11, an X-Y stage 13 for moving the tank 11 fixedly placed thereon in an X-direction and a Y-direction, that are orthogonal to each other in a horizontal plane, and a stage drive mechanism 14 for controlling the movement of this X-Y stage 13. The workpiece 7 is fixedly arranged on the table 12 in the dielectric 10, so that part thereof to be machined is positioned relative to the tool electrode 4 by means of the X-Y stage 13 driven and controlled by the stage drive mechanism 14.
The discharge circuit 5 in the control unit 3 comprises a capacitor 15 connected between the tool electrode 4 and the workpiece 7, and a resistor 16 and a power source 6 that are serially connected to each other but connected in parallel with respect to the capacitor 15. In this discharge circuit 5, a cycle is repeated wherein electric charges are charged at the capacitor 15 by applying voltage from the power source 6 through the resistor 16, and discharged when the tool electrode 4 is fed forward by the machining head 1 to a predetermined position in close proximity to the workpiece 7 with a small gap between the two and dielectric breakdown occurs.
Further, although not shown, the control unit 3 includes a CPU, and drive control units and the like for controlling the stage drive mechanism 14, as well as controlling the rotation of the motor 9, for moving the machining head 1 precisely.
Drilling of micro holes is accomplished by utilizing this electro discharge that is brought about when the tool electrode 4 is approached to the workpiece 7 with a certain gap therebetween by the downward movement of the machining head 1, in a state that voltage is applied from the discharge circuit 5 across the tool electrode 4 and the workpiece 7 via the dielectric 10. Machining proceeds as the workpiece 7 is melted by the heat accompanying the intermittent electro discharge between the tool electrode 4 and the workpiece 7, while dust or chips that are subsequently produced are removed. Thus, marks formed in the workpiece by the electro discharge eventually develops into a hole.
Electronic discharge machining apparatus of this type has various characteristic features as described below. Firstly, micro-machining is possible, by the minimization of discharge energy and by the miniaturization of the tool electrode 4. Workpiece 7 of any kind of material can be machined as long as it is conductive, and even materials having high hardness can be machined with ease. Further, materials having high specific resistance such as silicon or ferrite, which are susceptible to cracking in a normal EDM process, can also be machined. Secondly, since it is non-contact machining process, machining is performed to the workpiece 7 without subjecting it to a machining force. Moreover, machining can be performed highly precisely with respect to a curved surface, inclined surface, or a thin plate, which can hardly be achieved by a conventional drilling method. Furthermore, machining of a hole with a depth ten times greater than the diameter of the hole is possible, as long as the diameter is more than 50xcexcm, and thus it can favorably be applied to the machining of a mold for use in plastic processing such as punching of micro components.
In the EDM apparatus as described above, machining of a shape is achieved in such a way that the configuration of the tool electrode is transferred to the workpiece. Thus in the case of a cylindrical tool electrode, as the machining proceeds, a hole matching the shape of the tool electrode 4 is drilled. Accordingly, for machining a shape that is different from the distal end surface configuration of the tool electrode, the position of the workpiece 7 relative to the tool electrode 4 is changed in succession corresponding to the desired shape, by controlling the movement of the X-Y stage 13 driven by the stage drive mechanism 14, and by repeating each time the discharge machining.
For example, in the case of forming a tapered hole of which cross section reduces gradually toward the bottom, a hole having a cross section matching the cross section of the distal end surface of the tool electrode 4 is first drilled. Machining is then performed with respect to this hole, so that the hole will be tapered, with the diameter being maximum at the upper end. In order to widen the diameter of the hole that is already formed, the workpiece 7 is positioned relative to the tool electrode 4 so that the tool electrode 4 faces a position at the periphery of the hole, and the electro discharge machining is started from this point. The tool electrode is moved downward, while relative positions of the workpiece 7 and the tool electrode 4 are gradually changed such that, the portion being machined by the tool electrode is moved in succession toward the inner side of the hole. This process of machining from the upper edge of the hole down to the bottom is repeated until the tool electrode 4 has made one round of the hole, whereby a tapered hole having a largest diameter at the upper edge is machined.
In a conventional EDM apparatus, in cases where a large-size shape different from the configuration of the distal end surface of the tool electrode 4 is machined as described above, because electro discharge machining must be repeatedly performed as the position of the workpiece 7 relative to the tool electrode 4 is changed in succession in accordance with the machining shape, it takes considerable time for machining the entire shape. In addition, because electro discharge machining is started every time the position of the workpiece 7 relative to the tool electrode 4 is changed, there is the problem that machined surface roughness is deteriorated because, for example, precise roundness could not be achieved, or because marks are created on the machined surface in a stepped fashion, or because of generation of machining dust or burrs accompanied by excessive discharge.
The machining surface roughness of the workpiece 7 is also deteriorated when abnormal electro discharge occurs in the EDM apparatus described above. In the event of abnormal discharge, not only charging and discharging are repeated at the capacitor 15 in vain, and also, electric current flows out from the power source 6 through the resistor 16 across the short-circuited tool electrode 4 and the workpiece 7, which causes energy loss, and deteriorates the machining surface roughness.
To deal with such situation caused by abnormal electro discharge, measures are taken to cause the tool electrode 4 to retract from the workpiece 7 by a distance long enough to restore the electrically insulated state upon detecting short circuit between the tool electrode 4 and workpiece 7 based on the fact that the machined amount is less than a predetermined level. In this way, in the event of abnormal electro discharge, the operation time is protracted owing to the procedures for restoration to the normal state.
Moreover, in the conventional EDM apparatus, the peak current and discharge time during charging/discharging at the capacitor 15 are defined fixedly by the capacity of the capacitor 15 and the resistance of the resistor 16, because of which flexible control of discharge corresponding to the material of workpiece or various types of machining modes cannot be effected.
A further problem in the prior art is that, as the machining of a shape that is different from the shape of the tool electrode proceeds, the dielectric stagnates in the drilled hole in the workpiece, which leads to electrolysis between the workpiece and tool electrode via the dielectric. This happens particularly when ultra pure water is employed for the dielectric, and as the machining is performed for a prolonged period of time, the workpiece and the tool electrode are caused to be conducting, whereupon the electro discharge efficiency decreases, and precise machining of a desired shape can hardly be accomplished.
In view of the foregoing, an object of the present invention is to provide an electro discharge machining apparatus that is capable of machining a workpiece with improved surface roughness and in a reduced operation time.
To accomplish the above object, the present invention provides an electro discharge machining apparatus comprising:
a table for holding thereon a workpiece within dielectric;
an X-direction moving stage and a Y-direction moving stage for moving the table to a given position in X and Y directions that are orthogonal to each other on an identical plane;
a tool electrode arranged displaceable in a machining direction that is orthogonal to both of the X and Y directions for bringing about electro discharge between itself and the workpiece when brought to a position in close proximity to the workpiece;
a position detecting element for detecting a current position of the tool electrode in relation to the workpiece;
an operation circuit for calculating a next position to which the workpiece should be moved based on data regarding the current position of the tool electrode in relation to the workpiece obtained from the position detecting element, and predetermined machining data that are registered beforehand; and
a drive control unit for controlling movement of the X-direction moving stage and the Y-direction moving stage based on operation results of the operation circuit, wherein
the workpiece is automatically and continuously moved to a next position in the X and Y directions corresponding to the current position of the tool electrode in accordance with a machining shape.
The present invention also provides an electro discharge machining apparatus comprising:
a table for holding thereon a workpiece within dielectric;
a tool electrode for bringing about electro discharge between itself and the workpiece when brought to a position in close proximity to the workpiece;
an electrode drive mechanism for forwarding the tool electrode toward the workpiece;
a control unit for effecting control of the electro discharge brought about between the tool electrode and the workpiece, and for controlling the electrode drive mechanism, wherein the control unit comprises:
a discharge circuit for supplying current for discharge by discharging electric charges charged at a capacitor by a power source device through a resistor;
a current detecting element for detecting a discharge current and outputting a discharge current detection signal;
a discharge control circuit which detects occurrence of abnormal discharge based on a predetermined condition in which the discharge current detection signal is input, and shuts off power supply from the power source device.